Ethylene/vinyl alcohol copolymers containing from about 25 mol % to about 45 mol % ethylene have excellent properties for packaging applications. These polymers are normally produced from the corresponding ethylene/vinyl acetate copolymers. The presence of residual acetate moieties adversely affects the barrier properties of ethylene/vinyl alcohol copolymers. Therefore, it is necessary to reduce the acetate groups to as low a concentration as possible. Conditions for producing polyvinyl alcohol from polyvinyl acetate are well-documented in the literature, but low residual acetate levels are not usually very important for polyvinyl alcohol. Two mol % acetate is commonly considered to be a high degree of hydrolysis. In addition, the solubility characteristics and reactivity of ethylene/vinyl acetate copolymers are quite different from those of polyvinyl acetate. Accordingly, different technology is needed to produce ethylene/vinyl alcohol copolymers with the required low content of residual acetate. Furthermore, in order for these polymers to have the thermal stability required for melt processing, it is important to produce polymer with very low residual catalyst residues. The basic catalysts that are typically used also catalyze decomposition reactions that result in discoloration, gas evolution, and gelation at melt processing temperatures. Because these catalysts are generally alkali metal salts (e.g., NaOH), the ash content after combustion can be taken as a measure of the level of catalyst residues present in the polymer. High ash levels are correlated with poor thermal stability. This invention provides a process for the preparation of ethylene/vinyl alcohol copolymer with very low residual acetate content and ash content.
Patents of interest include the following:
U.S. Pat. No. 3,560,461 describes a process for the production of ethylene/vinyl alcohol copolymers by countercurrently contacting an ethylene/vinyl acetate polymer solution with alcohol vapor in an amount sufficient to affect the saponification and remove the acetic acid ester by-product as overhead from the reaction tower. Methanol is the preferred solvent, and a strong base catalyst is used. The examples illustrate the use of caustic soda at concentrations between 3.9 and 9.8% relative to the weight of the ethylene/vinyl acetate polymer. The products of the examples contain between 0.8 and 1.7 mol % residual acetate groups. This process is a continuous process requiring a pressurized column and high temperatures to keep the ethylene/vinyl alcohol polymers in solution. If the polymers precipitate during the reaction, buildup of the polymer in the tower is likely, and it is unlikely that very low residual acetate levels would be achieved. This patent makes no mention of neutralization of catalyst residues.
U.S. Pat. No. 3,847,845 describes a process for the separation and purification of ethylene/vinyl alcohol polymers. The process includes mixing water with an anhydrous methanol soution of an ethylene/vinyl alcohol polymer after the saponification has reached the desired degree of completion. The resultant solution is extruded into an aqueous coagulation bath to form a microporous sheet or strand which is then washed with water to remove catalyst residues. The sheet or strand is then dried. In this process, the polymer is dissolved and the resulting solution is extruded. Catalyst residues are removed by washing the sheet without any prior neutralization. The polymer is isolated by precipitation in a coagulation bath.
U.S. Pat. No. 4,041,223 relates to a process for preparing hydrolyzed ethylene/vinyl acetate polymers having improved thermal stability. The process includes a first-stage reaction in methanol to a degree of hydrolysis of greater than 90 mol %. A secondary hydrolysis in water is used to achieve a degree of hydrolysis greater than 97 mol %. An acid may be added to the polymer suspension to neutralize the catalyst. The ethylene/vinyl alcohol polymer is isolated between the first and second-stage reactions and the methanol content is reduced to less than 15% by weight before slurrying in water. An acid may be added to the suspension to neutralize residual catalyst after completion of the reaction. The polymer is then contacted with a solution of a phosphoric acid to provide further stability. This process includes a drying step between the first and second stages. The examples demonstrate degrees of hydrolysis between 99.0 and 99.5% after the second stage. The polymer is produced in the form of a powder after drying.
U.S. Pat. No. 4,367,305 describes saponified ethylene/vinyl acetate polymers improved in behavior in the molten state. The improvements are achieved by immersing the polymer in a solution of water-soluble manganese and/or copper salts and an acid with a pKa between 3.5 and 6.5 to produce a polymer with between 0.00005 and 0.005% by weight manganese or copper. The polymer must also give an extract with a pH between 3.5 and 6.5 when extracted with 100 mL of water per 20 g polymer. The examples discuss a saponification reaction conducted at 110.degree. C. in methanol under a pressure of 3.5 kg/cm.sup.2 while distilling off methyl acetate. The ethylene/vinyl alcohol polymer product has a saponification degree of 99.2%. Water is introduced after the reaction is completed and a paste-like liquid is extruded into a coagulation bath. The pellets are then washed and treated with dilute acetic acid and an aqueous solution of manganese (II) acetate and dried.